KX65 plug reading after full engine rebuild - with pics

mike-evans

Member
Sep 16, 2009
142
0
Whats your thoughts on the plug colour. Just run a full engine rebuild in, 2 heat cycles on the stand (5mins idle then cool completely), 15 mins half throttle then warm up and into bands gently.... 125ml putoline to 5 lites fuel (40:1). Standard jetting.

Thinking of dropping the needle 1 position and maybe 1 richer in the main?

Also, should I run the 32:1 ratio per the service manual?



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Okiewan

Admin
Dec 31, 1969
29,555
2,237
Texas
Until the resident guru gets here, here's some excellent information that you may find very useful.

Follow this link: "Sparkplug Reading 101" by Rich Rohrich
 

High Lord Gomer

Poked with Sticks
Sep 26, 1999
11,790
34
Just curious....why drop the needle *and* go richer on the main jet?

Dropping the needle (raising the clip to a higher notch) allows the needle to drop farther into the main jet and thereby leans the fuel mixture. Going leaner on the needle and richer on the main would be useful if it was running rich in the midrange but running out of fuel or lean on top.

I've never been able to read plugs for jetting. I had better luck by diagnosing the systems of how the bike runs and responds to throttle changes.
 

mike-evans

Member
Sep 16, 2009
142
0
What I mean is "drop" the clip on the needle, so richen it by one....

Ive made the main jet richer by 1 (from 190 to 200), and this is now the colour






Ill be making the neelde richer (dropping the clip), by 1 for tomorrows ride

Whilst out a few days ago, when the bike was hot, there was a lean bog when revving it (in neutral) from idel
 

Rich Rohrich

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Unfortunately you can't read a plug from running it the way you've described, there is way too much overlapping of circuits to pin point any area other than wide open throttle (WOT). You can reliably read the main jet and timing with a fresh plug and a loaded (like a slight uphill) WOT run for 10-20 seconds. It's important to try and chop the throttle at the end of the WOT run, and pull the plug. Part throttle running back to your pits just confuses things if you are trying to get an accurate plug reading.

All you can really tell from the pictures you posted is the plug is running a bit cold for the load you are putting on the engine and not burning off the deposits. This makes sense given that you were running it easy with a minimum of load. Richening the jetting to try and color the ceramic will just make it a slow burbling pig. The color on the nose of the plug merely indicates the plug running temperature. It may need more fuel but you can't say for sure by looking at those pics.

As Gomer suggested, you should change the jetting based on the way the engine responds for the low speed and mid throttle positions. You can not read the plug to determine the fuel curve in those areas, so it's not worth wasting your time. You can use a plug reading to determine WOT jetting but that's about it from a jetting standpoint. The plug can tell you a lot of interesting stuff about what is happening in the engine, but sadly low and mid throttle jetting isn't on the list. Fuel requirement is a function of load and throttle position, technically manifold pressure but that tracks pretty closely to throttle opening. That's why you can't really tell anything useful about your jetting by blipping the throttle on the stand.

In a crisp running engine a plug running at the correct temperature should look more like this. Whitish with just a touch of coloring :

ltm_9_noshell_drn-jpg.4932



The picture below is a plug running a bit too cool. The setup is safe safe but not as sharp as it could be. The plug pics you posted shows a lack of heat and that's what causes the coloring, not the jetting. As you add more fuel you will cool things down further in the chamber so it will add more color, but it isn't any indication of the jetting. An overly lean engine with too cold a plug can have coloring similar to the plugs you posted. All it's telling you is the plug isn't running hot enough to burn the deposits off of nose of the ceramic.
plugd_noshell-jpg.4930


Start rich on the pilot and needle settings and carefully lean them till the engine has the required throttle response you are looking for. Except in very special circumstances you can't hurt the engine by going too lean on the pilot or the needle setting. The worst thing that will happen is the engine will bog or feel flat.

Here's a link to Eric Gorr's Carb Tuning article. It's the best explanation of the process you are ever likely to find. Happy Tuning !

http://dirtrider.net/forums3/threads/carburetor-tuning-intro-by-eric-gorr.184616/
 
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mike-evans

Member
Sep 16, 2009
142
0
Thanks for the info Rich, I was hoping you would come along :-)

Im trying to play it safe and run the engine more rich that lean as I dont want to face another mechanical failure.......

Ive read many jetting guides and experimented when we was out this week. With the bike in third and the throttle barly cracked, I opened the throttle and without hesitation the engine opened right up - this tell me the mid circuit is fine?

The latest plug colour I posted is with a main jet increase from 190 to 200, bike running in 4th band and engine killed.

Was surprised to get a lean bog when the bike was hot and we took the bike back to the van and revved it in neutral, never ever had that before...

Im running a BR9EG plug but the maunal says to use B1OEG, should I use a B10EG?

I am surprised that the jetting needs altering after a full rebuild, but also it makes sense as the tight/fresh engine is now running more efficient?

I know youve stated that its impossible to read all the jetting circuits from the plug colour, but with the engine picking up without hesitation when in third and the current plug colour, would you say it is running richer versus lean?
 

Rich Rohrich

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Jul 27, 1999
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Thanks for the info Rich, I was hoping you would come along :-)

Im trying to play it safe and run the engine more rich that lean as I dont want to face another mechanical failure.......

Ive read many jetting guides and experimented when we was out this week. With the bike in third and the throttle barly cracked, I opened the throttle and without hesitation the engine opened right up - this tell me the mid circuit is fine?

The latest plug colour I posted is with a main jet increase from 190 to 200, bike running in 4th band and engine killed.

Was surprised to get a lean bog when the bike was hot and we took the bike back to the van and revved it in neutral, never ever had that before...

Im running a BR9EG plug but the maunal says to use B1OEG, should I use a B10EG?

I am surprised that the jetting needs altering after a full rebuild, but also it makes sense as the tight/fresh engine is now running more efficient?

I know youve stated that its impossible to read all the jetting circuits from the plug colour, but with the engine picking up without hesitation when in third and the current plug colour, would you say it is running richer versus lean?

If it feels sharp in the midrange without hesitation it sounds like you have the needle jetting pre spot on . Double check the main on a short WOT run with a clean plug and you should be good to go.

Don't worry about a no load bog on the stand, it doesn't mean much if the bike responds properly on the track. I would run the 9 series plug based on the pics you posted. If the plug is getting too hot you'll see signs of blistering on the nose and glazing of the ceramic where it looks like glass. It's hard for most riders to push a bike hard enough on an MX track to need a cold plug like a 10. That's more for running in sand all day or deep mud where the load is high at all points all day.
 

mike-evans

Member
Sep 16, 2009
142
0
Thanks again Rich :-)

As Im overly paranoid at this stage due to putting in a bunch of new parts and listening for every single noise, one other question I have is, how hot should the cylinder be to touch?

When we warm the bike up for a few minutes, the cylinder gets hot, as expected it does when the lad has been riding for a 20-30 minute session....

I cannot remember how hot the cylinder used to get before the rebuild
 

Rich Rohrich

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Your body temp is 98.6 so anything over 100 will feel hot to you. The radiator and cylinder wall will always be hotter than you can touch once the engine is warm. Unless it is boiling over I wouldn't worry about it.
 

mike-evans

Member
Sep 16, 2009
142
0
Rich calling on your expetise again :-)

Removed the pipe after yesterdays time at the track to look inside the cylinder, theres some rub marks at the rear if the cylinder, no depth to any if them.

Just removed the head to checl rest of cylinder and its all good.

Clean and oiled filter everytime. Bike warmed until the cylinder is warm then a steady lap until opened up - everytime.

Your thoughts?






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The white bits you can see is from tissues used to wipe
 

Rich Rohrich

Moderator / BioHazard
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Jul 27, 1999
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I think the surface finish of the bore, and the ring surface material would have a much greater influence than the type of oil used during break in.
 

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